Within a mail piece processing system, gap length is defined as the distance between two mail pieces, i.e., the distance between a first mail piece's trailing edge and a second mail piece's leading edge. In order for proper continuous function of a mail piece processing system, the gap length must be large enough to accommodate the time required for electromechanical devices (e.g., diverters, scales, printers, etc.) operable along the processing system's mail piece transport path to perform their functions.
As an example, in a mail sorter system, it is common to include a series of tightly positioned transport belts guided by one or more pulleys, actuators, rollers, tracks and the like to transport mail pieces from an initial feed position to an output position. Close contact between the belts and mail pieces enables the physical transport of the mail pieces. Between the input position and output position various other modules may also operate upon or interact with the mail pieces; for example, an imaging system for interpreting the markings resident upon the mail pieces or one or more scales for weighing each mail piece. A plurality of mail bins for accumulating the sorted mail pieces may be located beyond the output position. When one considers the plurality of modules and procedures that must be executed in order to direct mail pieces along the mail piece transport path at high speeds, it is evident that maintaining proper gap length between mail pieces throughout the transport path is critical. For example, if the gap length between mail pieces is too small, a diverter may not be able to divert a first piece of mail and recover in time to divert a second piece of mail or to let the second piece pass the diverter. This failure can lead to a mail piece not being diverted to its proper course or, more destructively, cause a system stoppage (e.g., due to jamming or mail pieces.)
Presently, gap length is controlled by the operation of the mail sorting system feeder at the front end of the system. Feeders operate using a set pitch; pitch being the distance between the leading edge of a first piece of mail and the leading edge of a second piece of mail. The pitch setting is generally established and controlled through the use of a processor/controller, which may regulate the timed release of mail pieces to affect the pitch, as well as control and monitor the various electromechanical devices of the sorter system. Knowing the length of the longest piece of mail fed to the feeder and operating at a set pitch allows for a minimum gap length at the output of the feeder. Alternately, a fixed gap feeder sets a fixed amount of time between detection of the trailing edge of the mail piece that just left the feeder and when the next piece is advanced out of the feeder. However, controlling gap length at the output of the feeder does not guarantee control of the gap length at all points along the mail sorting system.
The feeder is assumed to function correctly at all times, with no variation in output to the system. Unfortunately, feeders do not function perfectly at all times and it is common for gap length to vary in the output of a feeder. Stops and starts of the mail sorting system can create variations in gap lengths as certain pieces of mail may accelerate and decelerate at different rates based on the slickness of the mail pieces and belts, the thickness of the mail pieces, belt elasticity, etc. Also, gap length variations may occur due to variations in belt tension at certain points throughout the mail processing system, whether the tension variations are intentional or unintentional. For example, the belt tension (and hence hold) upon mail pieces may be intentionally lessened to allow said mail pieces to settle into a mail piece guidance track. In contrast, the belt tension may change unintentionally as a result of wear over time due to normal usage. Regardless of how it occurs, gap length variation is a common occurrence during mail processing system operation.
If mail sorting systems were able to monitor the variations in gap length along the mail piece transport path during the mail processing operations and alter one or more processes within the mail sorting system based on the variations, the mail processing system would be able to avoid costly stoppages and improve operating efficiency. Also, simply monitoring where variations in gap length are occurring could demonstrate that there is a particular point in the system that is known to cause variations in the gap length. This information could allow a system operator or monitor to identify problems in the system, for example, a failing bearing, a failing belt, a sticking point, etc.
Therefore, a need exists for a system and method in which the gap length and/or mail piece length is both measured, tracked and controlled instantaneously and at multiple positions along the mail sorting system.